I have a feeling this is destined for 'medical'.
Most of the type of problem you describe is postural, so your question about "how to sit" is very appropriate. Are these problems something you've had before flying? If no, when did the symptoms begin? Have you had any neck/back injuries before? May I ask (even approximate) how old you are?
Don't fly the 73, so can't answer specifically, but need more information to diagnose.
As for exercises, I'd recommend you visit your primary care physician. You may also want to ask for a physical therapy referral, if he/she doesn't recommend one (don't know how this may work in Denmark). The RPT can show you what type of strenghthening/stretching exercises to do along with correct posture, and more importantly, HOW to do them and HOW often to do them.
Hope you feel better soon!